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Old 10-05-2025, 14:03   #1
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Barely 5 knots !

I’ve installed a Yanmar 3GM30F w/ a Kanzaki gear box w/ a 2:61 ratio
in a Tartan 30. Previous owner removed a blown 2GM in it.The prop is a 10D 12pitch three fin @ 2650 +~ RPM I can barely get 5 knots out of her! Wide open. The prop appears to be the one used with the 2GM motor taken out. Any thoughts ? Since the wiring harness was a Yanmar from the previous motor I plugged in the new motor and she’s worked fine except for top end hull speed.
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Old 10-05-2025, 15:52   #2
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Re: Barely 5 knots !

I have a 3GM30F and the same transmission in my Pearson 30.
I get hull speed at 2000 RPM with a 14 x 10, 3 blade prop.
I had a prop shop match the prop to the boat and drive system. I bought a new prop when I did the new engine.

At WOT you should be getting 3600 RPM. Can you get 3600 in neutral?
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Old 10-05-2025, 18:08   #3
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Re: Barely 5 knots !

Years ago, Jim investigated having a prop re-pitched. I don't know if it makes sense for <singlemalt> to investigate this for a big bronze 3 blade, or not. There is a risk of destroying the prop, so it might be better to simply sell it on and buy a new one. Who will know is the place that re-pitches them; the guy whose rice bowl is filled by the sale of new ones, maybe not so much.

Good luck with it.

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Old 10-05-2025, 18:56   #4
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Re: Barely 5 knots !

I've opined this on many a prop thread on this forum.

Get ahold of the "Propellor Handbook" authored by Dave Gerr. I believe it is available online on this forum.
The book is a technical read and gives a step by step way to determine what engine, prop, etc, is best for your boat, etc, etc.

It's a bit of a complicated and long winded exercise to explain here, but most people expect little diesel engines to perform far better than they really are able to.

Diesel engines are designed to run optimally at a certain rpm, but a correctly propped diesel should be able to attain it's max rated rpm while underway.
If your diesel is rated at say...3,000 rpm max rpm. it should be able to attain that underway, if it doesn't and say only manages 2500 rpm, you are over propped. In the above example a 10x12 prop is a tiny little thing and one should not expect it to perform miracles. In the same breath, if the engine spools up to 3,000 rpm without a hiccup, you are likely under propped.
You can get a prop re-pitched, but you can't increase the diameter. a 10x12 prop is already oversquare so to speak, in other words the pitch exceeds the diameter. There comes a point where you can only expect to pitch up a prop so much. Attaining a certain speed does not necessarily mean the prop is pitched correctly if it can't hold that speed in an adverse wind/wave condition.

Before I get too long winded on the topic, 5 knots for a 30' boat is not bad. I would imagine the hull speed of the Tartan is likely around 6 knots or so, but to get that extra knot requires one to do some homework first.
The engine, transmission, prop size, etc, must all work in harmony with each other.

Get the book and spend some time reading and studying it and you will be able to solve your own problem.
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Old 10-05-2025, 19:28   #5
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Re: Barely 5 knots !

Welcome to the forum with your first post Singlemalt. The first 3 things I’d check would be
1. high idle and then max rpm under load ( should not be the same, under load should be, say 300 rpm less)
2. Rotation of propeller shaft ( gotta be opposite to engine in ahead)
3. Is the clutch slipping. Check with a photo tach ( or get the strobe tach app for your phone) check oil temp.
There are 7 pages of instructions in my service manual regarding the shift lever adjustment and travel length, there’s a reasonable chance it’s out of adjustment, there are also 3 different gearboxes for your 3GM.
What speed did the boat achieve with the 2GM? ……10”D x 12”P is way smaller than what I’d expect your propeller to be.
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Old 10-05-2025, 19:33   #6
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Re: Barely 5 knots !

Here’s what Yanmar suggests.
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Old 10-05-2025, 19:35   #7
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Re: Barely 5 knots !

First thing I would check is the hull and prop to see if its clean becuase if its got weed and barnacles then you won't exceed 5 knots.

Next we need to know what the maximum revs you can achieve in gear and finally what are you using to measure the speed? A paddle wheel or GPS. The paddle wheel could by miles out if its not clean.

That is a tiny prop probably limited by hull clearance but also limits the amount of power you can apply to a prop. They can be altered by 1" of pitch btw.

Peter
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Old 11-05-2025, 09:56   #8
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Re: Barely 5 knots !

If you're in salt water, or even brackish water, you might want to make sure the hull is clean, along with the prop. Those devilish little barnacles can really
slow you down.
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Old 11-05-2025, 10:23   #9
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Re: Barely 5 knots !

You have 27 hp with a 2.6:1 gear ratio in a sailboat that only needs 18 hp maximum and a 2:1 gear ratio. Your prop is 10" diameter. That diameter will efficiently accept about 12 hp with 2:1 ratio. Without changing your engine or gear ratio, your only recourse is to fit the maximum diameter prop (probably 12") with the most blade area, Example: Michigan DJ355, using as much pitch as it takes. I would not go over square.

Talk to a prop shop. Give them all the specs on the vessel and engine.

The best engine for that boat would be 16 - 18 hp with a 2:1 ratio gear and turning a 12" diameter x 9" pitch, 3 blade propeller.

Over powering and over gearing a propeller on a displacement vessel always works against you. Will it work? Yes. But efficiency and performance will suffer.
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Old 11-05-2025, 10:41   #10
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Re: Barely 5 knots !

Over powering isn't the issue here, the boat will just reach hull speed before the engine is at full power.

But I agree that if the maximum prop diameter is that limited then the prop needs to spin faster, so less gear reduction would be better.
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Old 11-05-2025, 12:17   #11
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Re: Barely 5 knots !

just for funsies I googled the Tartan 30.

Engine specs vary from the old time Atomic 4...a gas engine to a variety of 2 cylinder 15-18 hp diesels..

27 hp definitely seems to be a tad overkill.

Not sure what the blade clearance is on these boats, but before considering a larger diameter prop, I'd be checking that.
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Old 11-05-2025, 12:45   #12
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Re: Barely 5 knots !

Propeller diameter much too small, 16 to 17 inches is more appropriate by about 10 pitch.

Check your hull clearance for accommodation of larger size diameter.

Review the Yanmar guidance.
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Old 11-05-2025, 15:54   #13
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Re: Barely 5 knots !

IMHO.... too small in dia. and too steep in pitch . Having the numbers backwards might be closer!? �� I have a 13"x9 pitch 3 blade on my 27' 8000+#, Beta 20/ rated 16 at cruise with a Kanzaki (?) 2:1 . Been told I'm near two inches pitch over propped . So over that I can hardly make 1900 rpm max , out of supposed 2600, and almost hull speed as the bow drives down. But, boy, is she quick off the line! ��

Seriously....investigate/all around to prop shops, owner's groups and OEM dealers to get the right specs

FW & FS
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Old 11-05-2025, 17:15   #14
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Re: Barely 5 knots !

Seriously, the Tartan 30 has a LWL of around 24'.
Using conventional theory this equates to a max. hull speed of around 5.9 knots.
So cruising at 5 knots is certainly a respectful speed.
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Old 11-05-2025, 18:12   #15
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Re: Barely 5 knots !

Quote:
Originally Posted by MicHughV View Post
Seriously, the Tartan 30 has a LWL of around 24'.
Using conventional theory this equates to a max. hull speed of around 5.9 knots.
So cruising at 5 knots is certainly a respectful speed.
I think it’s a bit more, maybe 6.5 or more. My 22.5’ water line yields a 6.35 kts more or less.
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